The present invention relates to a continuously variable traction roller transmission.
One of previously proposed continuously variable traction roller transmissions is disclosed, for example, in JP-A 61-124764. This continuously variable traction roller transmission comprises an input rotating disc connected to an input shaft, an output rotating disc disposed to face the input rotating disc, a pair of rollers disposed to contact opposed surfaces of the rotating discs for transmitting rotation of the input rotating disc to the output rotating disc, a pair of retainers for rotatably supporting the rollers through eccentric shafts each having a pair of eccentric and parallel rotation shaft portions, and a pair of tie rods for supporting the retainers and guiding movement in the direction of tangent lines of the rotating discs and rotation in the direction of movement so as to tilt the rollers and change a contact position thereof with the rotating discs, each tie rod being supported by a support pin arranged in the opposed direction of the rotating discs to be rotatable about the support pin. Input torque operates to press down one of the rollers, and press down the other thereof. The other roller produces an upward offset. When input torque operates this offset roller, each eccentric shaft is swung on the rotation shaft portion thereof supported by the retainer which is rotated simultaneously about the tie rod as a support shaft for rotating the roller, thus achieving decelerating action. Upon deceleration action, the retainer is moved upward by swing of the eccentric shaft. In accordance with upward displacement and rotational displacement of the retainer, a cam connected to the retainer is operated to move a shift control valve through the cam. Thus, a hydraulic piston for vertically driving the retainer undergoes hydraulic pressure against upward movement of the retainer. When force due to hydraulic pressure balances with upward force of the retainer, an offset of the roller becomes null, so that a shift ceases. Likewise, the one roller produces a downward offset, and carries out similar deceleration action.
As to the known continuously variable traction roller transmission, however, even if the shift valve does not operate, an increase in input torque causes a shift on the deceleration side due to elastic deformation of a disc, etc. Moreover, the pair of eccentric and parallel rotation shaft portions of the eccentric shaft are disposed to have a positional relationship where the retainer has less upward and downward movement due to swing of the eccentric shaft, so that when actuating the shift control valve to produce within the hydraulic piston hydraulic pressure against upward force of the retainer, it is necessary to rotate extra on the deceleration side the cam for feed backing movement of the retainer to the shift control valve, resulting in extra deceleration. Thus, the speed ratio is changed with torque variations, resulting in a difficult logic of shift control, or engine over-rotation.
It is, therefore, an object of the present invention to provide a continuously variable traction roller transmission which enables a reduction in variation of the speed ratio due to torque increase.